The present invention relates generally to an apparatus for regulating the current flow in an electric circuit and more specifically to a fault tolerant apparatus for regulating the current flow in an electric circuit in which optical coupling is used between the regulating apparatus and the regulated electric circuit.
AC/DC converters/ inverters used in high voltage direct current (HVDC) transmission systems operate at hundreds of kilovolts above ground. These converter/inverters use thyristors as regulating elements. The thyristors are connected in series/parallel arrangements in order to reliably switch the large voltages/currents involved. Light triggered thyristors and optical coupling between the thyristor control system and the thyristors provide the required electrical isolation between the converter/inverter circuits and the thyristor control circuits.
If a single thyristor in a series string of thyristors fails to conduct (fire) when triggered by the control circuits, the result is a very high voltage buildup across the mis-fired thyristor which can cause the mis-fired thyristor to fail. Similarly, a misfired thyristor in a parallel combination results in excessive current in the remaining paralleled thyristors. A single failed thyristor increases the stress on each other thyristor of the thyristor series/parallel combination and could result in total converter/inverter failure. Snubber circuits are used in converter/inverter systems to protect thyristors from damage due to voltage and current transients resulting from misfiring. However, it is desirable to prevent the misfiring from occurring, because even with snubber circuits, the additional stress placed on the thyristor by mis-firing can lead to deterioration of the thyristor and premature failure. Hence, the reliability of the optical source for providing a reliable optical triggering signal is of great importance for such switching devices, particularly when used in extremely high voltage circuits.
At the present time the main source of HVDC converter/inverter system failures using light triggering of thyristors is the light source itself. The majority of HVDC converter/inverter systems currently deployed use cesium lamp light sources for triggering converter/inverter thyristors. Cesium light sources are bulky and require the lamp to be always on, producing an undesired source of background light. Moreover, cesium lamps provide a large spectrum of light that is not fully used by the thyristor. The introduction of laser diodes as light sources has assuaged some of the problems resulting from use of the cesium light source but laser diodes still present a reliability issue in the total HVDC system design.
Accordingly there is a need for a light triggered thyristor apparatus which reduces the occurrence of thyristor mis-firing and is reliable, modular, small in size, low in cost and not excessive in power dissipation.